Change samplerVariable() to return a const char *.

This allows backends to use an expression rather than a variable for
sampler access.

Change-Id: I2346ac418c8d6277416a4430e2eaf6ab3ffee0a2
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/222036
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Stephen White <senorblanco@chromium.org>
diff --git a/src/gpu/glsl/GrGLSLProgramBuilder.h b/src/gpu/glsl/GrGLSLProgramBuilder.h
index 654dcbb..7563a92 100644
--- a/src/gpu/glsl/GrGLSLProgramBuilder.h
+++ b/src/gpu/glsl/GrGLSLProgramBuilder.h
@@ -51,7 +51,7 @@
 
     void appendUniformDecls(GrShaderFlags visibility, SkString*) const;
 
-    const GrShaderVar& samplerVariable(SamplerHandle handle) const {
+    const char* samplerVariable(SamplerHandle handle) const {
         return this->uniformHandler()->samplerVariable(handle);
     }